1. Field of the Invention
The present invention relates to an optical fiber cable equipped with a support wire used for an aerial line and the like.
2. Description of the Related Art
As a related art optical fiber cable, a structure comprising a support wire part and a main cable body part, which are continuously connected to each other, has been known, as described in Japanese Patent Laid-Open No. 115755/1998. FIG. 4A is a cross-sectional view showing an example of the related art optical fiber cable. In FIG. 4A, numeral 40 denotes an optical fiber cable, 41 denotes an optical fiber core forming an optical fiber ribbon stack, 42 denotes a hard resin covering for uniting the optical fiber ribbon stacks, 43 denotes an optical unit comprising the optical fiber core 41 and the hard resin covering 42, 44 denotes an outer sheath of a main cable body part covering the optical unit 43 in intimate contact therewith, 45 denotes a main cable body part, 46 denotes a tension member, 47 denotes an outer sheath of the tension member 46, 48 denotes a support wire part comprising the tension member 46 and the outer sheath 47 of the tension member 46, and 49 denotes a connecting part for connecting the cable main body part 45 and the support wire part 48. FIG. 4B is a side view showing the same optical fiber cable. The main optical fiber cable body part 45 is connected to the support wire part 48 in such a manner that the main cable body part meanders to have a slack with respect to the support wire part 48, and therefore when the optical fiber cable 40 is elongated on laying or by a strong wind, it is difficult to apply tension to the optical fiber core 41. However, an allowable elongating ratio is about 0.05% at most.
In order to fabricate an optical fiber cable that can be used under an environment in which strong tension is applied to the optical fiber cable, the larger slack should be given to the main cable body part with respect to the support wire part. For such a constitution, it is necessary that the main cable body part be connected to the support wire part through the connecting part with meandering on the surface thereof at a shorter interval. The constitution is not preferred from the standpoint of optical transmission characteristics, productivity and appearance, and also causes a problem in that the area receiving the wind is increased to further accelerate the elongating due to the wind pressure.
Under these circumstances, in an optical fiber cable, a practical amount of slack of the optical fiber main cable body part is limited to about 0.05% at most with respect to the support wire part.
Upon aerial laying of an optical fiber cable, a greater interval between the poles on which the optical fiber cable is hung, is required for decreasing the frequency of installation of the poles to reduce the operation of hanging the optical fiber. Therefore, an optical fiber is required to withstand strong tension applied by increasing the distance between the poles.
Along with the recent spread of optical communication networks, the amount of optical fiber cables laid aerially has also increased. Under these circumstances, there are often cases in that an additional optical fiber cable is laid with an optical fiber cable previously laid. Upon laying the additional optical fiber cable, there increase cases in that a ladder is set on the optical fiber cable, and the optical fiber cable is laid with strong tension being applied due to poor conditions of lying. Therefore, an optical fiber that can be used under stronger tension is required.
The related art optical fiber cable is constituted in such a manner that the optical fiber cores do not adhere to the inside of the hard resin covering, so as not to apply a local stress to the optical fiber core. In other words, the optical fiber cores are contained in the cable under the conditions which allows the optical fiber cores to move freely in the longitudinal direction in the cable. Accordingly, when the laid optical fiber cable is elongated, there arises a problem in that the optical fiber cores move in the longitudinal direction with respect to the outer sheath of the main cable body part, and an end of the optical fiber cores is dragged inside the main cable body part. In this case, there arises a problem where an excess length of the optical fiber core for joint is dragged inside to make the jointing operation difficult, and there are other problems where the jointed optical fiber core is broken by elongating, and the loss of transmission is increased by bending the optical fiber core in a small diameter. In the case where tension is temporarily applied to the optical fiber cable on setting a ladder or on laying operation, the dragged optical fiber core tends to return to the former state after releasing the tension, but the dragged optical fiber core is locally fixed due to the friction between the optical fiber core and the surroundings, causing the optical fiber core to buckle rather than return to the former state. In some other cases, such a fluctuation of the tension applied to the optical fiber cable causes the optical fiber core to move in the longitudinal direction in the optical fiber cable, resulting in dragging at one end and protruding at the other end.
An object of the invention is to provide an optical fiber cable with an end of an optical fiber core that prevents it from being dragged inside a main cable body part upon application of tension on the optical fiber cable, and that prevents the increase of transmission loss and breakage of the optical fiber core caused by movement of the optical fiber core near the connection point.
The invention relates to an optical fiber cable comprising a support wire part having a tension member, and a main cable body part, which are continuously connected to each other in a longitudinal direction, the main cable body part comprising a core comprising plural optical fiber cores, yarns stranded in a periphery of the core, and an outer sheath provided directly outside the yarns, and at least a part of the yarns being adhered to the outer sheath intermittently or continuously in the longitudinal direction.
In the optical fiber cable of the invention, it is preferred that a packing density of the yarns is adjusted to an amount which allows the optical fiber cores to exhibit a dragging amount of 30 mm or less upon the application of tension such that the optical fiber cable having a length of 80 m shows a elongating ratio of 0.2%.
It is also preferred that a packing density of the yarns is adjusted to an amount which allows the core to exhibit a withdrawing strength of 20 N or more upon application of tension such that the optical fiber cable having a length of 10 m shows a elongating ratio of 0.2%.
In the optical fiber cable of the invention, the core having the yarns stranded in the periphery thereof may be installed in a straight form in the outer sheath having a hollow shape, and the yarns may be installed in such a manner that when tension is applied to the optical cable, the yarns tighten the core to increase the withdrawing strength of the core.
According to the optical fiber cable of the invention, the breakage and increase in transmission loss caused by dragging the optical fiber core do not occur even under the conditions where the optical fiber cable shows a large elongating ratio of 0.2%, to ensure high reliability. Therefore, the invention provides advantages where the optical fiber cable can be laid between poles set apart at large intervals, and can be laid by high-speed traction, in which strong tension is applied to the optical fiber cable. Furthermore, in comparison to the related art optical fiber cable comprising an optical fiber core covered with a hard resin covering, the optical fiber core can be easily taken out after removing the outer sheath, to provide excellent operability of joint and processing of an end of the cable.